Untreated waste gases generated in the manufacture of trinitrotoluene contain tetranitromethane and various nitrogen oxides as pollutants. The prior art procedure was to scrub these waste gases with water and concentrated sulfuric acid, removing the nitrogen oxides but not the tetranitromethane. The treated waste gases, still containing tetranitromethane, were then discharged into the atmosphere. However, it has been determined that tetranitromethane is an undesirable pollutant which should be removed from the waste gases before they are vented to the atmosphere.
Although processes for converting tetranitromethane into trinitromethane are known in the prior art, trinitromethane is still expensive and the supply is limited and erratic. Therefore, it would be desirable to develop an economical process for removing tetranitromethane from waste gases and converting it into trinitromethane, HC(NO.sub.2).sub.3, or its salts. For instance, one prior art process uses aqueous potassium hydroxide to reduce tetranitromethane into trinitromethane (nitroform) [see Schmidt, Ber. 52,400 [1919]]. The usefulness of this process is reduced, however, by the simultaneous hydrolysis of the tetranitromethane to potassium carbonate with the corresponding reduction in the yield of the desired trinitromethane product in accordance with the following equations: EQU C(NO.sub.2).sub.4 + 20H.sup.- .fwdarw. C(NO.sub.2).sub.3 .sup.- + H.sub.2 O + NO.sub.3 .sup.- [I] EQU c(no.sub.2).sub.4 + 60h.sup.- .fwdarw. co.sub.3 .sup.-.sup.2 + 4no.sub.2 .sup.- + 3h.sub.2 o [ii]
the relative yields of the two sets of products formed in these competing reactions depends on the concentrations of tetranitromethane and potassium hydroxide. Unless very high concentrations (over 7 normal) of potassium hydroxide are used, the yield of trinitromethane will be less than 80 percent of the tetranitromethane present in the solution. In tests using an aqueous 0.41 N sodium hydroxide solution as a scrubbing solution, only about 50 percent of the tetranitromethane was removed from waste gases; 75 percent of the tetranitromethane removed was converted into trinitromethane and 25 percent into carbonate ions. This corresponds to the reaction: EQU 4C(NO.sub.2).sub.4 + 12 OH.sup.- .fwdarw. 3C(NO.sub.2).sub.3 .sup.- + 4NO.sub.2 .sup.- + 3NO.sub.3 .sup.- + CO.sub.3 .sup.-.sup.2 + 6H.sub.2 O [III]
thus, the process inherently produces a low yield of trinitromethane. Moreover, the efficiency (amount of tetranitromethane removed from the waste gases) of an aqueous alkali metal hydroxide solution is low because of the low solubility of tetranitromethane in water. For instance an aqueous sodium hydroxide solution of from about 0.4 N to 0.5 N removes only about one half of the tetranitromethane present in the waste gases (see example I). In summary, the use of an aqueous alkali metal hydroxide solution alone is inefficient because of the low solubility of tetranitromethane in water and the competing reaction which converts tetranitromethane into carbonate ions.
In conclusion, it would be desirable to provide a process which would be more efficient in removing tetranitromethane from waste gases and in converting the tetranitromethane into trinitromethane.